The present invention relates to torque-transmitting apparatus having a friction facing material operably engageable with an opposing surface material in the presence of a transmission fluid or oil. More particularly, the invention relates to an improved friction facing material including a fluorocarbon polymer and to methods of preparing such friction facing materials for use in torque-transmitting apparatus.
The torque-transmitting apparatus comprises clutch and brake assemblies of the disc and opposing plate type. The friction facing material is secured to a metallic core to provide the friction or torque-transmitting surface of the disc. The opposing plate provides a cooperating surface which operably engages the friction material for purposes of torque transmission. A plurality of discs having friction facing material secured on each of the opposite surfaces thereof are typically interleaved with a plurality of opposing plates to provide a multiple disc torque-transmitting apparatus. Torque transmission is regulated by closing means which control the axial proximity of the adjacent discs and plates.
The disc and opposing plate may extend into a reservoir of transmission fluid, or the fluid may be delivered to the disc under pressure from such reservoir or from a remote reservoir. The fluid serves to cool the apparatus by dissipating the heat energy resulting from torque transmission, this being referred to as "wet operation" of the unit. The fluid may also serve to transmit torque by the shearing of films of fluid between adjacent discs and plates, as well as to dissipate heat, this being referred to as "hydroviscous operation" of the apparatus.
The heavier duty torque-transmitting apparatus and applications of concern herein are of the type encountered in large road vehicles, such as buses and trucks as well as off-the-highway and construction vehicles. In order to meet the torque loading requirements of such applications, friction facing materials, including major proportions of elemental carbon in a resin binder matrix, have been developed. For example, U.S. Pat. No. 3,261,440, assigned to the assignee herein and incorporated by reference, discloses friction facing materials containing elemental carbon in amounts exceeding 50% by weight. (All percentages are by weight unless otherwise indicated.)
The performance of such friction facing materials has been further improved through the use of epoxy resin binder materials and additives. However, these friction facing materials have not been entirely satisfactory, especially when used in combination with certain transmission fluids known to have high plating-out characteristics which are believed to be associated with decay of the dynamic coefficient of friction. The plating-out phenomenon refers to the formation of thin films of organic material on either or both of the cooperating torque-transmitting surfaces.
The torque transmission characteristics are determined by a number of factors, including the particular transmission fluid and the friction facing material as well as the nature of the cooperating opposing plate surface. Further, the torque transmission characteristics tend to vary with continued use and wear of the friction facing material, as well as the occurrence of plating-out of the oil, as noted above.
The maintenance of a stable dynamic coefficient of friction over the wear life of the friction facing material is desirable, since it tends to assure that the same energy transmission and performance also occur over the life of the system. Further, a stable coefficient affords smooth operation and tends to avoid distinctive initial operating characteristics during a "break-in" period. This is of particular concern in heavier duty applications, such as large vehicles or high load machinery.
Heretofore, certain combinations of friction facing material, opposing surfaces, and oil have provided a reasonable degree of stability of the coefficient throughout the wear life of the device. However, even in such cases, these satisfactory operational characteristics may be lost by the end user's replacement of the transmission oil with an oil which does not enable the synergistic effect of the combination but is otherwise acceptable.
The prior art teaches the use of fluorine-substituted elastomeric materials to provide friction facing materials in torque-transmitting apparatus. U.S. Pat. Nos. 3,261,440 and 4,042,085 are representative of such prior art teachings. These fluoro-elastomers have not provided entirely satisfactory performance.
Other elastomeric materials, such as nitrile rubber, have not been found to provide reproducible results when incorporated into high carbon content friction facing materials of interest herein. These problems have been accompanied by the added difficulties of rubber-type processing including formulation with the use of Banbury mixers and similar devices in an attempt to provide acceptable mixtures of additives.